1. Field of the Invention.
The present invention relates to a centrifugal separator for separating liquid and gas from a mixture or suspension of liquid and gas being fed into the separator.
Such separators may comprise a container having a circumferential wall shaped substantially as a surface of revolution with a substantially vertical axis. When a mixture of gas and liquid is introduced in the container and caused to rotate about said vertical axis, liquid and gas are separated under the influence of centrifugal forces so that gas may be discharged from the central part of the container through a discharge tube arranged at the upper end of the container, and liquid may be discharged at the lower end of the container adjacent to its circumferential wall.
However, in cases where the liquid in the liquid-gas mixture is inclined to foam, problems arise in connection with the known centrifugal separators. Thus, the central gas whirl in the separator influences the liquid in the separator so that foam is created, and this foam tends to be sucked into the gas whirl and discharged from the separator together with the gas phase so that the gas phase discharged will contain liquid particles. The remaining part of the foam created is discharged together with the liquid phase so that the liquid discharged from the separator contains gas and this may under certain circumstances give rise to problems.
Such mixture of gas and a liquid inclined to foam may, of course, have been created in any of widely differing manners or may originate from widely differing processes. There may also be several different reasons why it is desired to separate the mixture into its single phases. However, as an illustrative example it may be mentioned that the drying gas discharged from a spray drying system for producing milk powder or whey powder quite often has to be freed from its content of entrained dried powder. This may be done by injecting a liquid -- for example the milk or whey which is subsequently to be dried in the spray dryer -- into the discharge gas whereafter the mixture of discharge gas and liquid is passed to the centrifugal separator from which the separated, purified gas may be discharged into the atmosphere whereas the liquid separated therefrom and enriched with the powder washed out from the discharge gas may be passed further on to an evaporator. By introducing the liquid subsequently to be dried in the spray dryer into the drying gas discharged therefrom, a heat economical advantage may be obtained. This is especially true when the drying gas discharged from the spray dryer has a relatively great drying potential which is the case for example in spray drying of whey. When skimmed milk is being dried the drying potential of the discharge gas is somewhat smaller.
When known centrifugal separators of the above type are used for purposes as that mentioned above as an example, the creation of foam and the sucking of this foam into the central gas whirl are especially disadvantageous because it may cause that conditions for bacterial growth are provided in certain parts of the separator. Furthermore, a substantial content of gas bubbles in the liquid discharged from the separator may give rise to problems when the liquid is passed to an evaporator as mentioned above.
2. Description of the Prior Art.
German "Auslegeschrift" No. 1,168,868 discloses a cyclone having an outer wall part formed as a downwardly narrowing truncated cone in the lower end of which a cross-sectional constriction is defined, for example by a liquid discharge tube extending upwardly in the lower part of the cyclone, or by a disc-shaped insert mounted on a vertical stem or shank. In the said patent specification it is maintained that a cyclone of this type is able to defoam foaming sulphite lye to a substantial extent. In the patent specification it is admitted, however, that it may be difficult to obtain a complete separation of the liquid and gas phases, and that part of the separated liquid tends to move into the central gas whirl where the liquid is converted into foam which is mainly removed together with the gas phase. Furthermore, the said known separator is not suitable for treating food products, such as milk products, because in that case a fairly sharp separation between the dry and wet inner surface parts of the apparatus is necessary, and the residence time in the separator must also be relatively short for all parts of the liquid or foamed product so that no possibilities for heavy bacterial growth are created. The last mentioned condition is not fulfilled by the above mentioned known cyclone because its lower part has such a complicated shape that portions of the liquid or wet product may obtain a rather long residence time in certain areas of the separator.
U.S. Pat. No. 3,731,467 discloses a centrifugal separator of the previously mentioned type having a circumferential, cylindrical wall, and liquid discharge means which include level control means for securing a predetermined minimum liquid level at the lower part of the circumferential container wall. The end walls of this known separator container have the usual dished or domed shape in order to increase their strength. The liquid discharge tube of the known separator is connected to the separator container at a position spaced above the upper part of the domed bottom wall of the container. Consequently, during operation the separator container contains a relatively great amount of rotating liquid which totally covers the container bottom because the liquid level must necessarily be higher than the position at which the liquid discharge tube opens into the container. That means that the central core of the gas whirl formed in the separator will be immediately adjacent to the liquid supported by the container bottom, and thus liquid particles will tend to be sucked up into the whirl core. If the liquid is liable to foam the gas discharged from the container through the gas discharge tube will contain a substantial amount of foam.